My moral intuition is against pushing the fat man into the path of a trolley because, in real life, this isn’t going to stop the trolley. Now the question is framed so that you know that the death of the fat man will save the lives of the four other people, but even if that were so, I wouldn’t, in a real life situation, believe it.

The problem with utilitarianism is that it is not always possible to know the consequences of your actions. A secondary problem is that it usually can be used to find a way to rationalize unethical conduct. So it might be better, even from a standpoint of utilitarianism, for people to obey moral rules or moral intuitions rather than trying to calculate utility in doubtful cases.